1. Field of the Invention
The present invention relates to an electrically conductive composition used in the production of solar cells.
2. Background Art
Conventionally, a composition formed by dispersing an electrically conductive powder and glass frit in an organic vehicle has been employed as an electrically conductive composition (hereinafter referred to as a conductive paste) for forming thick film electrodes in electronic elements. Such a conductive paste is applied to a ceramic substrate, a ceramic element, etc. through a method such as printing, and the resultant product is then dried and fired so as to remove organic components and sinter the conductive particles.
In recent years, thick film electrodes have demanded low-temperature firing in order to save energy and lower cost. With regard to materials which can be fired at low temperature in air, a conductive paste containing Ag powder (hereinafter referred to as Ag paste) has often been used since Ag powder is relatively inexpensive and Ag has low specific resistance. However, necking for growth of Ag grains requires a certain amount of heat during firing and can thereby result in insufficient sintering, particularly when sintering is performed at a low temperature of 700.degree. C. or less. Therefore, desirable conductivity and film strength sometimes cannot be attained.
Meanwhile, an Ag paste containing Ag powder, glass frit and an organic vehicle is often used for forming electrodes of semiconductor elements such as Si solar cells. FIG. 1 illustrates a typical prior art Si solar cell. In the cell, an antireflection film 21 (TiO.sub.2) and Ag electrodes 25 are formed on the light-accepting surface of an Si wafer 23, in which a n.sup.+ /p/p.sup.+ junction has been formed, and an Al electrode 27 is formed on the back surface of the Si wafer 23. To obtain this structure, a Ag paste is applied onto the antireflection film 21 through screen printing, and fired in a near-infrared-radiation furnace. If the Ag electrodes 25 do not penetrate through the antireflection film 21 or do not establish ohmic contact with Si through an insulating film such as SiO.sub.2 formed on the silicon wafer 23, the contact resistance to Si increases and thereby deteriorates the fill factor (hereinafter abbreviated as FF) which is a factor of the V-I characteristics of a solar cell. In contrast, when a Ag paste is burnt at relatively high temperature, the contact resistance decreases to enhance the FF. However, in this case, components such as Ag and glass components diffused from the electrodes destroy the pn junction of the Si wafer to disadvantageously cause deterioration of voltage characteristics.
Generally, addition of Pb or Bi to an Ag paste is known to enhance sinterability of the Ag electrodes. These additive elements provide the effect of improving sinterability of Ag electrodes when firing is performed at a temperature as high as 700.degree. C. or more since these elements contribute to facilitation of Ag through self-vitrification. In another approach, Ag powder serving as a conductive component is finely divided in an effort to lower the sintering starting temperature. However, this approach is not practical, as it involves high costs.